1. FIELD OF THE INVENTION
The present invention relates to an electronic safety system, and specifically to a system for electronic safeguarding against burglary intended for use in stores, specifically in department stores, including a plurality of emitters and a plurality of receivers each defining respective emitter-receiver pairs intended to cooperate with a label equipped with a resonance circuit, the emitters each being operative to produce an electromagnetic field which excites the resonance circuit of the label if such label is located within the zone of the electromagnetic field from an associated emitter, the receivers each being operative to respond to electromagnetic signals from an associated emitter and electromagnetic signals from the label and to evaluate said signals and to trigger an alarm upon detection of a signal from the resonance label.
2. DESCRIPTION OF THE PRIOR ART
A multi-installation safeguarding system of this kind is disclosed e.g. in U.S. Pat. No. 4,667,185 (Nourse et al.). In the known system the different emitter-receiver pairs are synchronized with each other. The emitters are alternatively switched on to be operative and off to be inoperative. The receivers are only activated during quiescent intervals between emitted bursts. In U.S. Pat. 4,531,117 (Nourse et al.) a single installation system is disclosed in which the emitter is wobbled (swept) through a frequency range in which the resonant frequency of the resonance label is located. In the presence of a resonance label a characteristic deformation of the received signal is generated which is evaluated and leads to the triggering of the alarm. The receiver is designed narrow-banded and its frequency is always tuned to the emitter frequency.
In systems as mentioned above interfering signals from various sources superimpose themselves over the received signal from the label, and as the characteristic of the deformation caused by the label fluctuates within wide limits, it is very difficult to achieve a sufficient discrimination of the label signal (an intelligence signal) from the interference signals. The most significant interfering signals cannot be avoided in spite of filtering.
In the single installation system reflections and diffractions of the emitter signal itself occur which interfere with each other and with the original emitter signal. Because these interferences depend, due to the phase difference of superimposed signals, from the momentary emitter frequency, the receiver signal is also different for each frequency, particularly also in case of an absence of any label. Accordingly, a receiver signal is produced which differs distinctly from a constant (and only theoretically present) "rest signal" and looks confusingly like a label signal when the emitter is wobbled.
In addition to these interferences, similar interferences are produced in the multi-installation system by a superimposition of the signals of adjacent emitters (and their reflections).
In the single installation system the disturbing interferences can be eliminated by switching on and off the emitter during alternating time intervals and by indentifying the label only during the time intervals in which the emitter is inoperative (echo method). Alternatively, a background subtraction technique can be used for this purpose as well. In the multi-installation system this is, however, not sufficient even if the system is synchronized.